by Patrick L. Cahalan

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Exposure

That or a doozy of a screw-up photographically. More likely, this is a photo that was grossly underexposed so that all that came through was pure black.

Exposure is the heart and soul of the technical end of photography. Without it, no matter how great a shot you find and frame, you’ll have garbage after you capture the image.

Exposure is a combination of three things that determine how much light get to your camera’s sensor or your film.

ISO – this is how sensitive the sensor or film is to light. More sensitive (higher numbers) means more light will register.

Shutter speed – how long the light has to impact the sensor/film.

Aperture – how big the opening the light passes through is.

To get a proper exposure, these three settings must be correctly combined.

This is also where a bit of terminology is needed. You’ll see a lot about “stops” mentioned with regard to exposure. Basically, each “stop” of light is double the amount of light hitting the sensor as compared with the stop below it. So stops measure light in powers of 2 (or if you prefer thinking this way, in terms of doubling and/or halving the amount of light). You can change stops by changing any one of the three main settings.

You’ll also see aperture referred to as f-stops, or notated with things like f/5.6, f/22 and so on. This is measured on a logarithmic scale (i.e. it does not look like powers of two). The standard stops are, from most light to least, f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/14, f/22, f/32, and f/64. Each stop is half the light of the previous one.

Exposure time is generally measured in fractions of a second. So for example, 1/60 means one sixtieth of a second, ¼ means one quarter of a second and so on. This is explicitly powers of two: 1/50 means double the light of 1/100.

ISO is also measured explicitly in powers of two. Most DSLRs will start around ISO 100 or 200 (occasionally ISO 50), and go up from there. ISO 400 is twice as much exposure as ISO 200, and so on.

Bear in mind what I said at the outset … the three must be combined to get a good exposure. And in fact you can get the same amount of light with different settings! For example:

ISO 200, 1/200, f/5.6

ISO 200, 1/100, f/8

These are actually the same overall amount of light hitting the sensor. From the first line, I doubled the exposure time and halved the aperture width, which equal out. Or as another example:

ISO 3200, 10 sec, f/4

ISO 100, 320 sec, f/4

Here I cut the ISO by 5 stops and increased shutter speed by 5 stops – same amount of light. But you’ll see wildly different results on these two! The second one allows several minutes to go by, so moving objects may not even appear any more on the final image.

The good news is most modern DSLRs are outfitted with tools to help you, the most important being the light meter. This is seen in your camera’s head’s up display, and looks something like this.

In-camera Light Meter

As you alter the settings for exposure you’ll see the dash marks move around … you want them in the center, generally. Note that this is not 100% all of the time. Not all light meters are created equal, nor is all subject matter. For example, shooting at night or in infrared tends to be beyond the abilities of the light meter in my camera, which means more experimentation, experience, and finagling is in order.

For after-the-shot, you can also use the camera’s histogram to see how well the exposure came out. I will go into more depth on this in the next post rather than be over-long today.

Instead I’ll leave you with an image I think came out very well. This is one that I bracketed because I had a devil of a time getting the exposure right.

Moulton Barn, Mormon Row, Grand Teton NP

If you ever go here, don’t believe the map that all the roads are paved. Took me $20 in quarters at the car wash to get all the damn mud off my truck after driving that road 3 days after a snowstorm.